1. Field of the Invention
The present invention relates to a magnetic display panel that uses magnetic force to form a clear record display and to erase the display.
2. Description of the Prior Art
A conventional technique of forming a display using magnetic force uses the application of a magnetic field to a dispersion system having fine magnetic particles dispersed in a colored liquid dispersion medium to cause the fine particles to migrate and change the color of the dispersion system. According to this technique, a magnetic field is applied with a magnetic pen to a dispersion system composed of a dispersion medium colored with a dye or pigment in which is dispersed fine magnetizable particles of a color different from that of the dispersion medium. The degree by which the migrating particles are shielded by the dispersion medium is varied to change the color of the dispersion system.
This technique unfortunately suffers from the following defects. With the dispersion system sealed between two opposing substrates, when a magnetic pen is moved over the surface of one substrate to apply a magnetic field to the dispersion system, the magnetizable fine particles are attracted to scanned area of that substrate to form a letter or pattern that follows the locus of the pen. However, ones of the attracted particles having a specific gravity much greater than that of the dispersion medium precipitate away from the display surface with the lapse of time making it impossible to retain the letter or pattern over an extended period of time. Another defect lies in the fact that all magnetic particles within the influence of the magnetic field of the magnetic pen are attracted to the pen and hence even particles quite remote in distance from the pen are attracted thereby forming a greatly blurred display of letter or pattern.
If the precipitation of magnetic fine particles is prevented by using micro-fine particles or by coating the particles with a sufficient amount of a resin of low specific gravity to bring the apparent specific gravity of the particles close to that of the dispersion medium, the magnetic force acting on the magnetic particles is decreased to such a level that they become less attractable by the magnetic pen and fail to form a letter or pattern of high contrast and tone. In addition, letters or patterns formed with such particles are difficult to erase, because the particles are not easily attracted to the rear substrate even if a relatively strong magnetic field is applied to that substrate. After repeated recording and erasure operations, the fine magnetic particles tend to remain suspended in the dispersion system thereby darkening the system. This also makes the desired display and erasure characteristics unobtainable.
For the reasons set forth, no magnetic display panel had been commercially produced before the present inventors developed the display panel disclosed in U.S. Pat. No. 4,143,472 issued on Mar. 13, 1979. The magnetic display panel therein disclosed has two substrates between which are confined a dispersing liquid having a yield value of 5 dyne/cm.sup.2 or more and which is composed of fine magnetic particles, a colorant, a dispersion medium and a fine particulate thickener.
As described above, a magnetic display panel using a dispersion system having fine magnetic particles dispersed in a colored liquid dispersion medium has serious defects. The concept of that invention is based on our finding that a magnetic display panel using a plastic dispersing liquid having a yield value of 5 dyne/cm.sup.2 or more prepared by incorporating a fine particulate thickener in a dispersing liquid having fine magnetic particles dispersed in a dispersion medium provides a sharp display of high contrast. The display remains stable over an extended period of time and can be erased completely whenever necessary.
To maintain fine magnetic particles of high specific gravity in given positions in a liquid dispersion and to move the particles all at once upon application of a magnetic field, neither the control of the viscosity of the liquid dispersion nor the use of a dispersion stabilizer or protective colloid nor making the specific gravity of the dispersion medium equal to that of the magnetic particles is sufficient. The desired result can only be obtained with a dispersing liquid having a certain critical range of yield value. Our continued studies have revealed that such a result is obtainable with a dispersing liquid the yield value of which is controlled to be not less than 5 dyne/cm.sup.2 using a fine particulate thickener. If the yield value of the dispersing liquid is less than 5 dyne/cm.sup.2, not only is an indistinct display produced but also the fine magnetic particles rapidly precipitate after the display is formed. As a result, the displayed patterns may be indistinct or entirely illegible, and moreover will fade out with time. Such a drawback is most conspicuous when a dispersing liquid having a yield value of 0 dyne/cm.sup.2, that is, when no fine particulate thickener is used. The problem is reduced as the yield value is increased by the addition of a fine particulate thickener. Only when the dispersing liquid has a yield value of 5 dyne/cm.sup.2 or more is a magnetic display panel free from the defects mentioned above obtained. Therefore, it is critical to the invention of U.S. Pat. No. 4,143,472 that the dispersing liquid used have a yield value of 5 dyne/cm.sup.2 or higher.
The term "yield value" as used in that patent and herein means the lowest stress required to cause a liquid to flow. This is indicated by the stress at point A in the flow curve shown in FIG. 1.